PU.1 is one of the proteins that interacts with the GATA-1 DNA binding domain and there is reciprocal inhibition between the two proteins. The PU.1 transactivation (TAD) and DNA binding domains are reported to be involved in the interaction and we have noted that the PU.1 TAD shares structural homology with the TAD of p53. In collaboration with the laboratory of Jim Omichinski we have shown that the p53 TAD also interacts in vitro and in vivo with the GATA-1 DNA binding domain. The linker and C-finger of GATA-1 are required for the interaction. The proteins reciprocally inhibit the transactivation activity of one another in an erythroid precursor cell line, 6C2. GATA-1 may be required to prevent p53 induction during the nuclear condensation and enucleation that precedes erythrocyte formation or during the polyploidization of megakaryocytes. In collaboration with Masi Yamamoto we are testing mutants of GATA-1 that do not interact with p53 in rescue assays in GATA1.05 cells and mice to determine the role of this interaction during hematopoiesis.